The regulation of acetohydroxy acid synthase (AHAS) activity and synthesis is proposed here as a potential model system for the study of control of dispersed genes and their products. Escherichia coli K-12 was used to obtain mutations that result in AHAS activity which is resistant to feedback inhibition by valine (AHASr). Multiple, unlinked loci produce AHASr activity. These loci appear to be independent of structural genes for AHAS isozymes. This study will analyze products of the genes which, when mutationally altered, produce a valine-resistant AHAS activity. Genetic and biochemical analyses will be made to determine whether AHASr arises from ilvG, which is inactive in K-12, from duplications of ilvG, or from changes in regulatory proteins that normally cause AHAS to be valine sensitive.